Modern military and sporting rifles may have receiver assemblies formed of upper and lower portions, removably attached to one another to facilitate takedown or field stripping for cleaning. The upper and lower portions may be pivotally connected to one another via a hinge pin located at one end of the receiver assembly, the other ends being secured together by a lug and cross pin combination. While this two part receiver design boasts numerous advantages, one disadvantage occurs when “play”, also know as “lost motion” or “lash” is present between the upper and lower receiver assemblies. The play may result from dimensional tolerances between interfacing parts as well as wear between relatively moving parts such as the hinge pin and its bearings or the cross pin and its lug. Both tolerance and wear may result in a fit between the upper and lower receiver portions which is less than tight, allowing relative motion (play) between the two parts. This play can adversely affect the accuracy of the rifle, and may also be perceived as an indication of poor quality of design and/or workmanship. There is clearly a need for a device which can reduce or eliminate play between upper and lower portions of a firearm receiver assembly.